Chapter 15: Heat Exchanger Design and Maintenance
This is the 15th article in our Vacuum Heat-Treatment Series. Heat exchangers are an important part of the vacuum gas-cooling system, yet most users do not understand the critical role these units perform or have a clear understanding of the type of maintenance required to keep them operating at peak efficiency.
Most heat exchangers used in vacuum furnaces are essentially fin cooling units (Fig. 1), which depend on the surface area of their coolers as well as the temperature of the incoming water supply to achieve a given cooling rate. Factors to consider when designing these units include the total surface area as well as the type and design of the individual elements that make up the surface since there can be many types of finned tubes with differences in shape, arrangement and relative dimensions between the fins and the tubes. By Dan Herring
Chapter 14: Diffusion Bonding, Eutectic Melting, Outgassing and Related Topics
This is the 14th article in our Vacuum Heat-Treatment Series. What follows is a discussion about problems that can occur during processing of parts, diffusion bonding, eutectic melting and unexpected or uncontrolled outgassing. In all cases, part quality may be compromised, so we must understand these phenomena and how to avoid these conditions.
Diffusion bonding is a solid-state joining process capable of bonding together a wide range of small and large metal and ceramic part combinations. In those instances, however, where our intent was not to join components together, diffusion bonding can be an unexpected problem. In vacuum processing, metal surfaces remain very clean and free of oxides. When these near-perfect surfaces are in contact with each other or other surfaces (baskets or fixtures), certain elements have a tendency to interact between these surfaces via solid-state diffusion (i.e. inter-diffusion of atoms across the interface). The result is that the parts “stick” together or stick to the baskets or fixtures, the equivalent of being welded on a microscopic level. In some cases the effect is minor and a slight tapping of the components separates them, and the surface “damage” is inconsequential. In other cases parts are fused together so strongly that the surfaces have to be literally ripped apart, ruining the components. By Dan Herring
Chapter 13: Cleaning of Parts and Fixtures
This is the 13th article in our Vacuum Heat-Treatment Series. What follows is a discussion of cleaning, one of the most important subjects in vacuum processing. Understanding the need for cleaning parts, fixtures and the cleaning system itself is critical to success, as is measuring how good a cleaning job we have done.
When vacuum furnaces were first introduced, many in the industry felt that the only acceptable part and fixture cleaning method was solvent vapor decreasing. Over the years, however, environmental and other factors have necessitated the use of aqueous systems. Therefore, it is important to understand how each method can successfully get the job done. Cleaning is the application of time, temperature, chemistry and energy to remove contamination from the surface of a part to a level appropriate for the intended application. In other words, cleaning is simply moving contaminants from where they are not wanted (on the parts) to where they should be (in the waste disposal system).
Cleaning is the application of time, temperature, chemistry and energy to remove contamination from the surface of a part to a level appropriate for the intended application. In other words, cleaning is simply moving contaminants from where they are not wanted (on the parts) to where they should be (in the waste disposal system). If all four aspects of the cleaning process are not working together, the parts will not be properly cleaned. Vacuum heat treating demands a high level of cleanliness compared to other methods; contamination left on parts can cause significant problems both in the equipment (Fig. 1) and on the parts themselves. Most vacuum systems are required to operate below 1 torr (1.33 mbar) and as such cannot contain or introduce any contaminants with a vapor pressure (at the process temperature) near the operating vacuum pressure. As is often the case, more than one contaminant is present, so the sum of the vapor pressures of each will be the limiting pressure of the system. By Dan Herring
Chapter 12: Leak Rates, Leak Detection & Leak Repair
This is the 12th in a series of articles in our Vacuum Heat-Treatment Series. What follows is a discussion about acceptable leak rates, leak detection and leak repair methods used on most vacuum vessels. Controlling the leak rate is of major importance in producing sound metallurgical components.
A common problem experienced by almost every vacuum user is that, over time, leaks develop that are both damaging to product quality and to furnace internal components. In extreme cases, the problem is obvious: the furnace will not pump down and/or the hot zone (or heating elements) shows obvious signs of oxidation. Small leaks, which are more common, often go undetected because the pumping system can overcome any air infiltration. However, even small leaks can cause continuous and sometimes catastrophic damage. Thus, routine leak checking should become a part of any good vacuum furnace maintenance program. By Dan Herring
Chapter 11: Vacuum Valves, Penetrations, Feedthrus and Flanges
This is the 11th in a series of articles in our Vacuum Heat-Treatment Series. This part discusses vacuum valves, penetrations and flanges found on most vacuum vessels; where they are used, how they operate and a little about how they should be maintained.
Valves intended for vacuum service are subjected to a variety of special conditions (Fig. 1), ranging from high and ultrahigh vacuum levels to low, high and ultrahigh pressures, differentials in pressure and differentials in temperature as well as variable frequencies of mechanical operation. They can be supplied in a number of configurations: ball valves, gate valves, butterfly valves, needle valves, isolation valves, pressure-relief valves and control valves just to name a few. The type of valve in use is typically identified by its design or function, and each type can be actuated in a variety of ways (manually, electro-magnetically, pneumatically, electro-pneumatically or via electric motor). Position indicators and limit switches located on the valves are common. By Dan Herring
Chapter 10: Partial Pressure, Mean Free Path & Related Topics
This is the 10th in a series of articles in our Vacuum Heat-Treatment Series. This part continues a discussion begun in Part Seven (Vapor Pressure) and focuses on the use of partial pressure and related areas necessary to control vaporization and prevent damage to both the parts and the equipment.
One of our goals in vacuum furnace processing is to minimize both alloy depletion from the part surface and subsequent hot zone contamination. Many of the materials we run are processed at temperatures and pressures at which individual elements can volatilize (leave the part surface). Partial pressure systems (Fig. 1) are designed to prevent this from happening by establishing a combination of pressure-temperature-time that minimizes the vaporization of the more volatile alloy constituents. By Dan Herring
Chapter 9: Heating Elements
This is the ninth in a series of articles in our Vacuum Heat-Treatment Series. This part talks about heating elements used in vacuum furnaces, the materials and temperatures of operation, forms and maintenance practices. The design and location of the heating elements is critical to achieve proper heating and uniformity of temperature.
Almost all high-temperature vacuum furnaces are electrically heated. Resistance heating elements are constructed from metal or graphite in a variety of styles. In general, one of the following materials is used:
- Stainless steel alloys – 300 series alloys (e.g., 304L, 316L) can be used for heating elements up to approximately 760°C (1400°F).
- Nickel/chromium and iron-aluminum based alloys – These typically operate up to temperatures of 1150°C (2100°F) and exhibit good-to-excellent oxidation resistance, making them useful for a number of applications including hot wall-type furnaces.
- Inconel® and other nickel alloys – Depending on material and vacuum level, they can be used up to 1150°C (2100°F). Above 800°C (1475°F), there is a risk of evaporation of chromium from these materials.
- Silicon carbide (SiC) – These elements have a maximum operating temperature of 1090°C (2000°F). There is a risk of evaporation of silicon at high temperatures and low vacuum levels of less than 0.133 mbar (100 microns).
- Molybdenum – With a maximum operating temperature of 1700°C (3100°F), molybdenum becomes brittle at high temperature and is sensitive to changes in emissivity brought about by exposure to oxygen or water vapor.
- Graphite – These elements can be used up to 2000°C (3630°F). Graphite is sensitive to exposure to oxygen or water vapor, resulting in reduction in material thickness. The strength of graphite increases with temperature.
- Tantalum – Elements made of tantalum have the highest duty temperature, typically 2400°C (4350°F). Tantalum, like molybdenum, becomes brittle at high temperatures and is sensitive to changes in emissivity brought about by exposure to oxygen or water vapor. by Dan Herring
